1. Field of the Invention
The present invention relates generally to aircraft automatic flight control systems and more specifically to speed control and acceleration to a higher speed during the climb portion of flight.
2. Description of the Prior Art
Most commercial transport aircraft, general aviation aircraft and military aircraft are equipped with an automatic flight control system. Automatic flight control systems generally provide the pilot with the capability of altering the flight path of the aircraft to achieve and maintain a desired speed, measured either in knots or Mach number.
During a climb portion of flight, the pilot may elect to increase the speed of the aircraft through a manually-entered speed command or may be mandated by Air Traffic Control (ATC) to accelerate to a specified speed. Soon after liftoff, the pilot will generally increase the speed of the airplane in order to retract the flaps. In addition, in the United States, the Federal Aviation Administration (FAA) requires that aircraft speed be no greater than 250 knots indicated airspeed at altitudes less than 10,000 feet. Thus the pilot is generally required to accelerate the aircraft in successive steps as the aircraft configuration changes and minimum altitudes are reached. Further, it is desirable to accelerate to the optimum climb speed as quickly as possible in order to maximize fuel savings. In addition, a positive rate of climb should be maintained at all times while accelerating and it is generally an ATC requirements that a minimum climb rate of 500 fpm will be maintained, particularly at low altitudes.
In the prior art, these accelarations were generally accomplished by the well known method of decreasing the pitch attitude of the aircraft in a manner proportional to the difference between the new speed command and the actual speed of the aircraft, or speed error. While this scheme will accelerate the aircraft to the desired Mach or airspeed, there is no assurance this will be done in a timely or optimal manner or that any minimum altitude rate restrictions will be met.
The present invention overcomes the shortcomings of the prior art by computing an altitude based upon the present climb rate, or altitude rate of the aircraft, and then increasing the altitude at a rate that will assure a 500 fpm rate of climb or less if the plane is not capable of accelerating at a climb rate of 500 fpm. This altitude is then used to tend to pitch the aircraft down to achieve the specified rate with the engine(s) at climb thrust until the difference between the desired speed and the actual speed of the aircraft is within a predetermined amount, at which time conventional speed control proportional to speed error is resumed.